One of the most common Mercedes 126 repair challenges is a high idle. Generally, the culprit is electronic, but sometimes a simple mechanical adjustment of the throttle linkage will solve the problem. Of course, older engines are likely to suffer from intake vacuum leaks of varying severity, and these also can cause idle problems. We shall look at vacuum leaks in another article.

ELECTRONIC CAUSES OF HIGH IDLE

The idle control module, located behind the false bulkhead next to the ABS control unit, is the most common cause of problems. The module processes a variety of inputs and controls the voltage sent to the idle control valve, which physically regulates the amount of air supplied to the intake ports whenever the throttle valve is closed. It is definitely useful to have a known working unit available for diagnostic purposes. While there is no one area of the circuit board that we can point to as a frequent location of cracked solder joints, there are four capacitors which tend to die with age. New units cost well over $200 and even rebuilt units are pretty expensive. I strongly suspect that, in most cases, all the rebuilders do is replace the capacitors.

The idle control valve can suffer from two problems. The electrical solenoid can fail or, more commonly, the valve simply becomes gummed up with oily deposits. (Some of the crankcase ventilation fumes are routed through the valve.) A good soak in carb cleaner will usually cure this ailment. Be careful not to expose the electrical part of the valve to the solvent.

Very rarely, there is an issue with the lambda control unit, the closest thing these cars have to an ECU. The idle control unit is connected to the lambda controller and “looks for” signals therefrom. Usually, if there is an issue with lambda control the engine will be running poorly anyway (due to incorrect fuel mixtures). If the lambda controller is not functioning at all, the most common reason is a failure of the over-voltage protection relay (OVP) located in the fuse box. There may also be a poor ground connection. The unit itself appears to be extremely reliable, possibly because it lives in a cooler climate — next to the front passenger’s feet.

If the idle problem occurs only when in one particular gear, the starter-lockout switch on the side of the transmission may be faulty.

If the problem occurs only when the A/C is used, there may be trouble with the delay relay in the fuse box, which is intended to boost engine revs when the compressor engages, thereby stabilizing idle speed under heavier accessory loads.

Finally, the throttle position switch (TPS) on the side of the throttle body may be defective. This is very unusual and we should be grateful for that, since replacement of the TPS requires removal of the whole intake manifold. Sometimes the switch is simply out of adjustment and can be tapped back into the right position with a long punch.

The TPS informs the idle control unit when the throttle valve is closed, partially open, or wide open. Electrical continuity exists across one pair of three pins, across another pair, or not at all. If the correct signals are not being sent, the switch itself might be perfectly fine; the problem may simply be that the throttle linkage is out of adjustment. The switch is extremely sensitive; even the slightest opening of the throttle is enough to break continuity across the pertinent pins. So it doesn’t take much excess pressure from the linkage to disrupt the system. Ensure that the throttle valve is in fact closing against its stop.

One other basic factor to consider – and one which tends to be neglected in this context – is your ignition timing. Too much ignition advance will most definitely have an effect on idle speed. On the Gen I cars, there is a lot of scope for adjustment, and the engines can handle quite a bit of advance before they start to detonate (ping). Around ten degrees of initial advance is optimal. Bear in mind that the vacuum advance unit adds about 16 degrees of advance at idle (when manifold vacuum is high). Occasionally, the advance unit can cause problems, the main one being a rupture of the internal diaphragm and creation of a vacuum leak at that point.